Last time I looked, the minimum legal length for a rifle was 26 inches. My XM177E2 clone, built on a Colt SP1 carbine, w/a 12inch barrel and a welded on suppressor to make a 16.1 inch total barrel, measures 29.1 inches with the stock collapsed.
It's not overall length that's the problem here.It's that the clowns that wrote the law, and the bigger clowns tha enforce it, have decided that you're a criminal if you have a stock that can be made longer or shorter----but only on a reciever that was made after a certain date.
See, if you have enough money to be able to afford a pre ban rifle, then you must not be a crook, cause you have enough disposible income to afford this kind of weapon. BUT, if you can only afford a less expensive, newer built unit, then they have to keep you from having a length adjustable stock, for you will surely use it to rob people with---so you will have enough money to buy the expensive preban, legal, version. Make sense? Not to me!
To the original question, if ATF certifies this "adjustable" stock as being OK for post ban rifles, they will be opening up a very large can of worms. Because at that point they are making judgements not about whether or not a stock is adjustable, but how adjustable is OK; and the law as I understand it does not give them that discretion. They will also be inviting lawsuits by thousands of CAR and M4gery owners , saying that if one brand of adjustable stock is OK, then they should all be OK.
Course I could be totally wrong about all of the above
. It's been known to happen